An installed conductor may be buried, embedded in an airframe, enclosed within walls, or otherwise removed from direct access. The testing and evaluation of an installed conductor will often present a problem. For example, the location and path of the conductor may not be obvious to an inspector. This may make the use of time domain reflectometry (TDR), standing wave reflectometry (SWR), and other reflectometric inspection methods difficult to use effectively by rendering the inspector unable to accurately correlate information presented by the test equipment to the conductor itself.
Signal generators and similar devices may be used to inject a signal into the conductor. This signal then radiates from the conductor, and may be traced by a “sniffer.” A sniffer is a device that detects the radiated signal and, based on the detection of a signal, indicates by lights, a meter, a tone, and/or other means its proximity to the conductor. An inspector may then trace the path of the conductor using the sniffer.
Certain types of anomalies may be detected through the use of a sniffer alone. For example, the tone may abruptly cease when the sniffer reaches an open or a short (for certain types of signal generators and sniffers) in the conductor. Signal generators and sniffers do not, however, normally convey data about other types of anomalies, nor do they present information as to the electrical distance the sniffer has traversed, where electrical distance is the propagation delay, i.e., the time the signal has taken to traverse the conductor from a point of injection (where the signal generator injects the signal) to a point of detection (where the sniffer is).
Signals travel down a conductor at a propagation velocity VC, measured in “distance/time”. The propagation velocity VC of a conductor is in all cases less than the velocity of light in a vacuum, c, and usually not less than one-half the that value, i.e.: 0.5c≦VC<C. The propagation velocity VC of a particular conductor is a peculiarity of that conductor, although certain “standardized” conductors (e.g., RG-6/U coaxial cable) have known and predictable propagation velocities VC.
The propagation velocity VC of a conductor may be used to convert a propagation delay DXY, (i.e., the electrical distance, or the time the signal has taken to traverse the conductor from a point X to a point y) into a physical length LXY of the conductor between points X and Y in a direct and straightforward manner: LXY=DXY/VC.
Therefore, using a suitable test apparatus (e.g., a reflectometer) able to determine propagation delay DXY from a reference point X (e.g., a signal injection point) to an anomaly at a point Y of the conductor, the physical distance LXY from that reference point to an anomaly of a conductor may readily be determined if the propagation velocity VC of the conductor is known. The accuracy of the result depends directly upon the accuracy with which the propagation velocity VC is known.
In many cases, the propagation delay DXY may be determined accurately, but the propagation velocity is not known. Determining the physical distance LXY cannot then be done without first determining the propagation velocity VC. Without knowing the physical length LXY, an inspector has no direct way of knowing the location of the anomaly, Y, relative to the point of injection, X.
Determination of the propagation velocity VC depends upon the nature of the conductor being inspected, its routing, and the nature of the environment in which the conductor is embedded.